ABSTRACT
Cu(4), Ag(4), and Au(4) species adsorbed on an MgO(001) surface that exhibits neutral (F(s)) and charged (F(s) (+)) oxygen vacancies have been studied using a density functional approach and advanced embedding models. The gas-phase rhombic-planar structure of the coinage metal tetramers is only moderately affected by adsorption. In the most stable surface configuration, the plane of the tetramers is oriented perpendicular to the MgO(001) surface; one metal atom is attached to an oxygen vacancy and another one is bound to a nearby surface oxygen anion. A very similar structural motif was recently found on defect-free MgO(001), where two O(2-) ions serve as adsorption sites. Following the trend of the interactions with the regular MgO(001) surface, Au(4) and Cu(4) bind substantially stronger to F(s) and F(s) (+) sites than Ag(4). This stronger adsorption interaction at oxygen vacancies, in particular at F(s), is partly due to a notable accumulation of electron density on the adsorbates. We also examined the propensity of small supported metal species to aggregate to adsorbed di-, tri- and tetramers. Furthermore, we demonstrated that core-level ionization potentials offer the possibility for detecting experimentally supported metal tetramers and characterizing them structurally with the help of calculated data.
ABSTRACT
Single d-metal atoms on oxygen defects F(s) and F(s+) of the MgO(001) surface were studied theoretically. We employed an accurate density functional method combined with cluster models, embedded in an elastic polarizable environment, and we applied two gradient-corrected exchange-correlation functionals. In this way, we quantified how 17 metal atoms from groups 6-11 of the periodic table (Cu, Ag, Au; Ni, Pd, Pt; Co, Rh, Ir; Fe, Ru, Os; Mn, Re; and Cr, Mo, W) interact with terrace sites of MgO. We found bonding with F(s) and F(s+) defects to be in general stronger than that with O2- sites, except for Mn-, Re-, and Fe/F(s) complexes. In M/F(s) systems, electron density is accumulated on the metal center in a notable fashion. The binding energy on both kinds of O defects increases from 3d- to 4d- to 5d-atoms of a given group, at variance with the binding energy trend established earlier for the M/O2- complexes, 4d < 3d < 5d. Regarding the evolution of the binding energy along a period, group 7 atoms are slightly destabilized compared to their group 6 congeners in both the F(s) and F(s+) complexes; for later transition elements, the binding energy increases gradually up to group 10 and finally decreases again in group 11, most strongly on the F(s) site. This trend is governed by the negative charge on the adsorbed atoms. We discuss implications for an experimental detection of metal atoms on oxide supports based on computed core-level energies.
Subject(s)
Elements , Magnesium Oxide/chemistry , Metals, Heavy/chemistry , Models, TheoreticalABSTRACT
Adsorbed atomic C species can be formed in the course of surface reactions and commonly decorate metal catalysts. We studied computationally C adsorption on Pd nanoclusters using an all-electron scalar relativistic density functional method. The metal particles under investigation, Pd(55), Pd(79), Pd(85), Pd(116), Pd(140), and Pd(146), were chosen as fragments of bulk Pd in the form of three-dimensional octahedral or cuboctahedral crystallites, exposing (111) and (100) facets as well as edge sites. These cluster models are shown to yield size-converged adsorption energies. We examined which surface sites of these clusters are preferentially occupied by adsorbed C. According to calculations, surface C atoms form strongly adsorbed carbide species (with adsorption energies of more than 600 kJ mol(-1)) bearing a significant negative charge. Surface sites allowing high, fourfold coordination of carbon are overall favored. To avoid effects of adsorbate-adsorbate interaction in the cluster models for carbon species in the vicinity of cluster edges, we reduced the local symmetry of selected adsorption complexes on the nanoclusters by lowering the global symmetry of the nanocluster models from point group O(h) to D(4h). On (111) facets, threefold hollow sites in the center are energetically preferred; adsorbed C is calculated to be slightly less stable when displaced to the facet borders.
